Showing papers by "McGill University published in 1981"
TL;DR: In this article, the problem of sensitivity reduction by feedback is formulated as an optimization problem and separated from the problems of stabilization, and the feedback schemes obtainable from a given plant are parameterized.
Abstract: In this paper, the problem of sensitivity, reduction by feedback is formulated as an optimization problem and separated from the problem of stabilization. Stable feedback schemes obtainable from a given plant are parameterized. Salient properties of sensitivity reducing schemes are derived, and it is shown that plant uncertainty reduces the ability, of feedback to reduce sensitivity. The theory is developed for input-output systems in a general setting of Banach algebras, and then specialized to a class of multivariable, time-invariant systems characterized by n \times n matrices of H^{\infty} frequency response functions, either with or without zeros in the right half-plane. The approach is based on the use of a weighted seminorm on the algebra of operators to measure sensitivity, and on the concept of an approximate inverse. Approximate invertibility, of the plant is shown to be a necessary and sufficient condition for sensitivity reduction. An indicator of approximate invertibility, called a measure of singularity, is introduced. The measure of singularity of a linear time-invariant plant is shown to be determined by the location of its right half-plane zeros. In the absence of plant uncertainty, the sensitivity, to output disturbances can be reduced to an optimal value approaching the singularity, measure. In particular, if there are no right half-plane zeros, sensitivity can be made arbitrarily small. The feedback schemes used in the optimization of sensitivity resemble the lead-lag networks of classical control design. Some of their properties, and methods of constructing them in special cases are presented.
2,203 citations
TL;DR: The origin, termination, and length of axonal growth after focal central nervous system injury was examined in adult rats by means of a new experimental model and the regenerative potential of these central neurons seems to be expressed when the central nervous System glial environment is changed to that of the peripheral nervous system.
Abstract: The origin, termination, and length of axonal growth after focal central nervous system injury was examined in adult rats by means of a new experimental model. When peripheral nerve segments were used as "bridges" between the medulla and spinal cord, axons from neurons at both these levels grew approximately 30 millimeters. The regenerative potential of these central neurons seems to be expressed when the central nervous system glial environment is changed to that of the peripheral nervous system.
1,665 citations
TL;DR: Clinical Pharmacology and Therapeutics (1981) 29, 111–123; doi:10.1038/clpt.1981.18
Abstract: Clinical Pharmacology and Therapeutics (1981) 29, 111–123; doi:10.1038/clpt.1981.18
1,097 citations
TL;DR: In this article, the shape and terminal velocities of bubbles rising in viscous liquids have been determined for Morton numbers (M) greater than 4 × 10−3 and R 110.
Abstract: The shapes and terminal velocities of bubbles rising in viscous liquids have been determined. For Morton numbers (M) greater than 4 × 10−3 the drag coefficient and dimensionless bubble shape are functions only of Reynolds number (R). Shape regimes and terminal rise velocities have been correlated. The flow field around a rising bubble was visualized through the hydrogen bubble tracer technique. For M > 4 × 10−3 and R 110 the wake was open and unsteady. Streamlines for the flow were obtained by raising a cine camera at the same speed as the bubble and filming the H2 tracer bubbles. Results are presented for R < 150 and 7·4 × 10−4 < M < 850.
726 citations
TL;DR: Survival of Darwin's finches through a drought on Daphne Major Island was nonrandom and selection intensities are the highest yet recorded for a vertebrate population.
Abstract: Survival of Darwin's finches through a drought on Daphne Major Island was nonrandom. Large birds, especially males with large beaks, survived best because they were able to crack the large and hard seeds that predominated in the drought. Selection intensities, calculated by O'Donald's method, are the highest yet recorded for a vertebrate population.
540 citations
TL;DR: period-doubling bifurcations, in which the period of a regular oscillation doubles, were predicted theoretically and observed experimentally.
Abstract: The spontaneous rhythmic activity of aggregates of embryonic chick heart cells was perturbed by the injection of single current pulses and periodic trains of current pulses. The regular and irregular dynamics produced by periodic stimulation were predicted theoretically from a mathematical analysis of the response to single pulses. Period-doubling bifurcations, in which the period of a regular oscillation doubles, were predicted theoretically and observed experimentally.
509 citations
TL;DR: In this paper, selected samples of waste microbial biomass originating from various industrial fermentation processes and biological treatment plants have been screened for biosorbent properties in conjunction with uranium and thorium in aqueous solutions.
Abstract: Summary Selected samples of waste microbial biomass originating from various industrial fermentation processes and biological treatment plants have been screened for biosorbent properties in conjunction with uranium and thorium in aqueous solutions. Biosorption isotherms have been used for the evaluation of biosorptive uptake capacity of the biomass which was also compared to an activated carbon and the ion exchange resin currently used in uranium production processes. Determined uranium and thorium biosorption isotherms were independent of the initial U or Th solution concentration. Solution pH affected the exhibited uptake. In general, lower biosorptive uptake was exhibited at pH 2 than at pH 4. No discernible difference in uptake was observed between pH 4 and pH 5 where the optimum pH for biosorption lies. The biomass of Rhizopus arrhizus at pH 4 exhibited the highest uranium and thorium biosorptive uptake capacity (g) in excess of 180 mgig. At an equilibrium uranium concentration of 30 mgiliter, R. arrhizus removed approximately 2.5 and 3.3 times more uranium than the ion exchange resin and activated carbon, respectively. Under the same conditions, R. arrhizus removed 20 times more thorium than the ion exchange resin and 2.3 times more than the activated carbon. R. arrhizus also exhibited higher uptake and a generally more favorable isotherm for both uranium and thorium than all other biomass types examined.
497 citations
463 citations
TL;DR: In this article, it is argued that there exist complex Gestalts among environmental, organizational, and strategy-making variables, and that these Gestalt are relatively few and very different from one another both in terms of the scores of, and relationships among, variables.
Abstract: Approaches that suffer from specification error and unwarranted generalizations are predominant among students of organizational adaptation and have resulted in fragmented and conflicting findings. The simplistic assumptions about organizations implicit in these approaches constrain the predictive power of findings and often prevent insights. It is argued that there exist complex Gestalts among environmental, organizational, and strategy-making variables, and that these Gestalts are relatively few and very different from one another both in terms of the scores of, and relationships among, variables. The Gestalts are expected to allow a richer, more complex, and multifaceted characterization of the process by which organizations adapt and change. They may also constitute enough structure in the data to yield more predictive findings. Some methodological approaches for identifying Gestalts are discussed [1].
453 citations
TL;DR: In this paper, the authors discuss various properties of matrices of the type S = H − GE −1 F, which they call the Schur complement of E in A = E F G H The matrix E is assumed to be nonsingular.
364 citations
TL;DR: In this paper, the secondary flow generated by the corona discharge is not negligible, and strong flow interactions take place owing to the induced circulatory cells, and the calculated numerical results demonstrate close agreement with experiment.
Abstract: Experimental and theoretical studies were undertaken to investigate the particulatefree secondary flow interaction in the wire-type electrostatic precipitator. The secondary flow generated by the corona discharge is not negligible, and strong flow interactions take place owing to the induced circulatory cells. The calculated numerical results demonstrate close agreement with experiment.
TL;DR: The numerical properties of some methods for computing controllability, including the numerical rank of a matrix, the numerical stability of algorithms, the sensitivity of problems, and the scaling of problems are discussed.
Abstract: The numerical properties of some methods for computing controllability are used in an expository way to motivate a wider understanding of numerical computations. In particular, the numerical rank of a matrix, the numerical stability of algorithms, the sensitivity of problems, and the scaling of problems are discussed. A numerically stable algorithm is given for computing controllability, but it is pointed out that a measure of the distance of the given system from the nearest uncontrollable system would be more useful, and this appears to be an open computational problem.
TL;DR: It is shown that enkephalin and catecholamines are released together from primary cultures of bovine adrenal medullary chromaffin cells by nicotine in a Ca2+-dependent manner.
Abstract: The opioid peptides Leu-enkephalin and Met-enkephalin are stored intraneuronally in the brain where they are thought to act as neurotransmitters and/or neuromodulators. Evidence for their release from nerve terminals has come from biochemical and pharmacological studies in vitro with brain tissue slices and synaptosomes. Enkephalins also exist in the peripheral nervous system in nerve cell bodies and axon terminals in the gastrointestinal tract, sympathetic ganglia and adrenal gland. In the adrenal gland, high levels of enkephalins are present both in axon terminals of the splanchnic nerve and in the adrenal medullary chromaffin cells where they are stored together with the catecholamines in the chromaffin granules. Stimulation of the adrenal gland in vivo or the perfused gland in vitro causes release of catecholamines and enkephalins into the adrenal vein. However, it is not clear whether the origin of the released enkephalins is the adrenal medullary chromaffin cells or the enkephalin-containing splanchnic nerve terminals that innervate the medulla. We now show that enkephalin and catecholamines are released together from primary cultures of bovine adrenal medullary chromaffin cells by nicotine in a Ca2+-dependent manner.
TL;DR: Hayes and Abemathy as mentioned in this paper argued that America's economic malaise can be traced to a failure of the entrepreneurial spirit, a lack of technological innovation, and a shortsighted view of the future.
TL;DR: In this paper, gas hydrates were brought up for the first time from two holes (497, 498A) drilled during Leg 67 of the DSDP in water depths of 2360 and 5500 m, respectively.
TL;DR: In this correspondence, an operator is derived that finds the best oriented plane at each point in the image, which complements other approaches that are either interactive or heuristic extensions of 2-D techniques.
Abstract: Modern scanning techniques, such as computed tomography, have begun to produce true three-dimensional imagery of internal structures. The first stage in finding structure in these images, like that for standard two-dimensional images, is to evaluate a local edge operator over the image. If an edge segment in two dimensions is modeled as an oriented unit line segment that separates unit squares (i.e., pixels) of different intensities, then a three-dimensional edge segment is an oriented unit plane that separates unit volumes (i.e., voxels) of different intensities. In this correspondence we derive an operator that finds the best oriented plane at each point in the image. This operator, which is based directly on the 3-D problem, complements other approaches that are either interactive or heuristic extensions of 2-D techniques.
TL;DR: It is concluded that breast-feeding does protect against later obesity and attribute the conflicting results of previous studies to insufficient attention to methodologic standards.
TL;DR: A linear, and thus optimal, algorithm is exhibited for solving the hidden-line problem in two dimensions, a recurrent problem in computer graphics.
TL;DR: A Schur-type decomposition for Hamiltonian matrices is given that relies on unitary symplectic similarity transformations as discussed by the authors, which preserve the Hamiltonian structure and are numerically stable, making them ideal for analysis and computation.
TL;DR: In this paper, the concept of task interdependence was developed and integrated in the blackman and Oldham [1976] theory of job design to differentiate between initiated and received task interdependent according to the direction of workflow in relation to the job incumbent.
Abstract: In this article I develop the concept of task interdependence and integrate it in the blackman and Oldham [1976] theory of job design. I differentiate between initiated and received task interdependence according to the direction of workflow in relation to the job incumbent. Each of these dimensions includes the elements of scope, resources, and criticality. Experienced responsibility for one's own work outcomes is differentiated from experienced responsibility for dependents' work outcomes. Testable hypotheses derived from the elaborated theory are set forth.
TL;DR: It was concluded that acetylcholine must reduce the release of γ-aminobutyrate either by a direct action on inhibitory terminals or by inhibition of inhibitory interneurons.
TL;DR: In this article, a noninterfering technique has been used to measure the concentration of ozone in pairs of bubbles injected into a bed of inactive 390 μm glass beads fluidized by ozone-free air.
TL;DR: The renewing cells of the adult show in rapid succession some of the features occurring slowly during the life of nonrenewing cells: origin from embryonic-like elements, maturation, and senescence indicated by a fragile state before life's end.
Abstract: Cell populations may be classified into three groups: static, which do not divide in the adult (e.g., neurons), expanding, which may divide but at a decreasing rate with age (e.g., kidney cells), and renewing, which undergo active division throughout life (e.g., blood cells, intestinal and seminiferous epithelium). The first two groups consist of nonrenewing populations whose cells usually survive as long as the body itself. In contrast, the cells of renewing populations have a short life, of the order of 2 to 3 days in the intestinal epithelium of rodents and 7 to 8 weeks in their seminiferous epithelium. In the present work, the life of a nonrenewing cell—the proximal convoluted tubule cell of kidney—is compared to that of the renewing cells of small intestine and testis.
The proximal convoluted tubule cell arises during embryonic development, when it has few organelles, many free ribosomes, a pale nucleus with diffuse chromatin, and a large open-network nucleolus. As the cell differentiates, organelles accumulate while free ribosomes decrease in number, chromatin masses appear in the nucleus, and the nucleolus becomes rather dense. With old age, some of the cells show features interpreted as senescence, particularly a small dense nucleolus.
The short-lived columnar cells of small intestine in adult rats arise from stem cells present in the base of the crypts. These cells have features similar to those of the embryonic proximal tubule cell, particularly many free ribosomes and a large open-network nucleolus. The stem cells give rise to cells which ascend crypts and villi. Shortly after the cells reach the villus, they acquire a full set of organelles, suggesting maturity, while free ribosomes decrease in number and the nucleolus undergoes condensation. Meanwhile radioautographic tests indicate that the production by the cells of glycoproteins identified as intestinal enzymes reaches a high level. However, when cells arrive in the villus tip region, their activity decreases, as shown by reduced enzyme production, while the nucleolus appears atrophic. The cells are then lost to the lumen.
The cells of the seminiferous epithelium also arise in the adult from stem cells with embryonic features. In the course of a complex differentiation, the nucleolus becomes atrophic and disappears. Soon thereafter the fragile spermatozoon stage is reached.
Thus, the renewing cells of the adult show in rapid succession some of the features occurring slowly during the life of nonrenewing cells: origin from embryonic-like elements, maturation, and senescence indicated by a fragile state before life's end.
TL;DR: In this paper, a classification of perturbations of local Hopf bifurcation problems which do not satisfy the classical non-degeneracy conditions is presented, up to symmetry-covariant contact equivalence.
TL;DR: Three peaks with somatostatin-like immunoreactivity (SLI) were identified and the relative proportions were unchanged after treatment with 8 M guanidine hydrochloride and thioglycol and varied considerably from tissue to tissue.
Abstract: The present study was undertaken to partly characterize and compare the relative amounts of the different molecular forms of somatostatin in the major somatostatin-containing tissues and portal plasma of the rat. Immunoreactive somatostatin (IRS) was extracted from hypothalamus, cerebral cortex, neural retina, sciatic nerve, pancreas, stomach, duodenum, jejunum, ileum, colon, and portal venous plasma and fractionated on Sephadex G-50 columns equilibrated with 6 M urea. Three peaks with somatostatin-like immunoreactivity (SLI) were identified. Peak I had an apparent molecular weight of 14,000 daltons, (14K SLI), peak II coeluted with synthetic somatostatin-28 (3K SLI), and peak III coeluted with SRIF (1.6K SLI). The relative proportions of the three molecular forms were unchanged after treatment with 8 M guanidine hydrochloride and thioglycol and varied considerably from tissue to tissue. In the hypothalamus and cerebral cortex, approximately 70% of the IRS was present as 1.6K SLI, 25% as 3K SLI, and 5% as...
TL;DR: An O(n), and thus optimal, algorithm is exhibited for determining edge visibility under any of the three definitions of a simple polygon from one of its edges.
Abstract: In many computer applications areas such as graphics, automated cartography, image processing, and robotics the notion of visibility among objects modeled as polygons is a recurring theme. This paper is concerned with the visibility of a simple polygon from one of its edges. Three natural definitions of the visibility of a polygon from an edge are presented. The following computational problem is considered. Given an n-sided simple polygon, is the polygon visible from a specified edge? An O(n), and thus optimal, algorithm is exhibited for determining edge visibility under any of the three definitions. The paper closes with an interesting characterization of visibility and some open problems in this area.
TL;DR: In this article, a quantitative model is proposed, based on the dynamics of plastic waves which accounts for the transition from "normal" to "inverse" transient behaviour with increasing strain, and also explains the opposite effects of stress relaxation and specimen unloading on the restraining transients.
Abstract: Tensile tests were performed on seven commercial polymers at 22° C and at constant true strain rates of 10−4 to 10−1 sec−1. The constant strain rates were imposed on the minimum section of each sample with the aid of a diametral transducer, an exponential function generator and a closed-loop hydraulic testing machine. The polymers investigated were: high and low density polyethylene, polytetrafluoroethylene, polypropylene. polyvinylchloride and polyamide 6 and 66. True yield drops were observed in the rigid glassy polymers, whereas yielding was more gradual in the semi-crystalline or plasticized polymers. Strain rate change tests were also performed, during which one order of magnitude increases and decreases were imposed on the specimens. “Normal” transients were observed at small strains in the samples containing a rubbery phase, while the transients were of an “inverse” nature in the samples containing a glassy phase. With an increase in the strain at which the change was initiated, the “normal” transients changed in character to “inverse”. Transient tests were also performed in which straining was interrupted to permit a period of stress relaxation or of holding in the unloaded condition prior to the resumption of straining. A quantitative model is proposed, based on the dynamics of plastic waves which accounts for the transition from “normal” to “inverse” transient behaviour with increasing strain, and also explains the opposite effects of stress relaxation and of specimen unloading on the restraining transients.